Oscillators capable of generating only certain frequencies are frequently used in semiconductor electronics. An example of such oscillators are digitally controlled oscillators (DCOs). Such a digitally controlled oscillator is represented exemplarily in FIG. 1. In this figure, the digitally controlled oscillator 1 is supplied with a parameter sin which can assume only a finite number of discrete values. The parameter can be any physical quantity, such as current, voltage, capacitance, inductance, resistance and the like. The value that is assumed by this parameter is usually determined by the status (value) of a digital bus. In the following, the various possible values of the digital bus are designated as digital words, and denoted by D1, D2, . . . , DN. The set of all digital words is designated as S(D)={D1, D2, . . . , DN}. An output frequency of the output signal fOUT of the digitally controlled oscillator 1 is uniquely assigned to a specific value of the input parameter sin, i.e., to a specific digital word out of the set S(D). It follows from this that the digitally controlled oscillator can generate only a discrete set of output frequencies. In the following, these frequencies are denoted by f1, f2, . . . , fN, and the corresponding set of all possible frequencies is denoted by S(f). It is assumed that the frequency fi is assigned to the digital word Di.
In many applications, it may be the case that a frequency which is not included in the set S(f) is to be generated by the digitally controlled oscillator. An example of this is a closed-loop control circuit, in which the output frequency of the digitally controlled oscillator is to be regulated to a multiple of a certain reference frequency, for example through use of a phase-locked loop (PLL). This reference frequency or its multiples is/are generally independent of the frequencies that can be generated by the digitally controlled oscillator, and therefore generally does not/do not correspond with elements of the set S(f).
Hitherto, this problem has been solved by techniques which usually make extensive use of analog circuits. An example of this is N-frequency-divider synthesis, used for wireless data transmission. In this synthesis, an oscillator, in this case a voltage-controlled oscillator, is driven by analog control signals. The desired output frequency is generated, in that an N-frequency divider is used in the feedback path of the frequency synthesis device. Usually in this case, the module of the N-frequency divider is digitally controlled by a higher-order (≧2) delta-sigma modulator, in order to reduce disturbing components of the output spectrum of the synthesis device. This solution, however, requires the use of analog circuits, with the typical problems associated with same. An example of this are variations in the gain due to variations in power, voltage or temperature. Moreover, this solution cannot be directly transferred to digital frequency synthesis architectures, in which there is no feedback path.